This invention relates generally to electron beam devices and, more particularly, to a method and apparatus for facilitating the correction of axial astigmatism in electron probe forming systems.
The resolving power of the scanning image obtained by a scanning electron microscope, an electron probe microanalyzer, or other electron probe forming system, is essentially determined by the cross-section size of an electron probe irradiating a specimen surface. It must be as small as possible for observing a high resolving power image. Some of the factors determining the cross-section size of an electron probe are spherical and chromatic aberrations which are virtually uncontrollable as they are inherent in a given scanning electron microscope. Practical ways to form the smallest cross-section diameter of the electron probe in a given microscope comprise optimum focussing adjustment of an objective lens (or final stage condenser lens) and optimum compensation for astigmatism. The focussing adjustment technique has been studied enough, and automatic focussing devices create no particular problems. (See, for example, U.S. Pat. No. 3,937,959.)
The axial (on-axis) astigmatism is essentially caused by the nonuniformity of material of the lens magnetic pole piece, incompleteness of mechanical work and contamination of the aperture. To compensate axial astigmatism, generally a stigmator is employed. However, astigmatism compensation can only be effected by skilled operators and is difficult to accomplish for unskilled persons.
A method of compensating for astigmatism has been proposed in U.S. Pat. No. 4,162,403. According to the method disclosed therein, two quadrupole lenses in an XY type stigmator are supplied with first and second (horizontal and vertical) scanning signals, respectively, for modulation of the stigmator in synchronism with scanning of an electron beam over a specimen surface. Information signals generated from the specimen are applied as brightness modulation signals to a cathode-ray tube (CRT) that is scanned in synchronism with the electron beam scanning, thereby displaying an XY image map on the CRT. The X and Y bright line index markers are adjusted in conformity with the X and Y coordinates located at the center of the most sharp image area, so that the direct currents, the values of which correspond to the positions of the bright lines, are supplied to the quadrupole lenses.
With this method, the condition of astigmatism compensation is indicated as the X and Y coordinates on the image, allowing unskilled operators to effect optimum astigmatism compensation with relative ease. However, since it is necessary to select the coordinate position, or "spot", wherein the beam is correctly compensated, corrective operation is not possible or is difficult when an optimum corrected area is inside a large pattern with uniform brightness in the whole image. Accordingly, it is necessary to select the kind of specimen for astigmatism compensation.
It is, therefore, the main object of this invention to provide an improved method and apparatus for compensating for astigmatism in electron probe forming systems regardless of the kind of specimen to be observed.
Another object of this invention is to provide an improved method and apparatus for easily and accurately compensating for astigmatism in electron probe forming systems.